Wideband transmission line model for Archimedean spiral slot structures

Spiral slot structures appear in a variety of microwave applications such as circuit elements, transmission lines and broadband antennas. However, design methodologies for these structures that are accurate above the first resonance involve multi-line transmission line models - requiring difficult to compute multi-line characteristics. This study presents the methodology for a transmission line model for an Archimedean spiral slot structure that uses singly-coupled line characteristics, which can be found analytically, to approximate the multi-line transmission line. With this model, the first resonance and many higher-order resonant frequencies of the spiral structure are well predicted, enabling multi-frequency use of the spiral slot. The results of the model compare well with full-wave simulations and are validated with measurements.